Petroleum is produced from subterranean formations through wells penetrating the formation. Initially natural reservoir pressure may be sufficient to cause the oil to flow to the surface. As natural reservoir pressure declines due to production from the reservoir, various forms of artificial lift may be employed to bring to the surface the oil which flows into the well bores. This phase of production is usually termed primary.
Typically, during the primary production period of a reservoir only 10 to 30% of the oil initially in place may be economically recovered. As the production rate from a reservoir falls near the level at which it is no longer economic to operate the field, active steps, usually termed secondary, are taken to increase recovery. In some instances, secondary recovery techniques may be employed from the time of initial production from a reservoir to optimize recovery.
The most common form of secondary recovery is the process known as waterflooding. In a waterflood project, water is injected into the oil producing formation through injection wells, repressurizing the formation and sweeping oil which would not have otherwise been produced into production wells. Such a procedure will usually allow the economic production of an additional 10 to 30% of the oil originally in place.
As is readily seen, a reservoir may have as much as 80% of the original oil in place still unproduced after primary and secondary operations have terminated. The increasing scarcity of crude oil and its rising price have led to the development of various techniques intended to allow further production from reservoirs which have been depleted by primary and secondary operations. These operations aimed at enhanced oil recovery (EOR) from conventionally depleted reservoirs are generally termed tertiary.
Among the most promising of tertiary recovery methods are processes wherein a solution of surfactants is injected into the reservoir. The surfactant solution mobilizes the oil remaining in place after conventional production and allows it to be swept into production wells.
To date most enhanced oil recovery surfactant systems have used a petroleum sulfonate or a blend of two or more petroleum sulfonates. Typically, the average equivalent weight of a petroleum sulfonate or the blend of petroleum sulfonates useful in enhanced oil recovery operations is about 400. A common technique is to use a blend of a water soluble petroleum sulfonate and an oil soluble petroleum sulfonate. The correct combination of the two petroleum sulfonates yields a stable micellar solution which lowers the interfacial tension between the crude oil and surfactant solution and is capable of mobilizing the residual oil in the formation. This form of chemical or surfactant flooding is called low tension flooding.
Another form of chemical or surfactant flooding utilizing petroleum sulfonates employs a phase partitioning surfactant system. Such a system operates through the formation of a middle phase microemulsion comprised of both the surfactant and crude oil. Phase partitioning surfactant systems generally require higher surfactant concentration than low tension systems and a cosolvent such as a C.sub.3 to C.sub.8 alcohol.
Still another form of chemical flooding utilizes so called oil soluble surfactant systems. Such systems are well known and are typified by Marathon Oil Company's Maraflood.TM. and Union Oil Company's soluble oil process.
A combination of increased interest in enhanced oil recovery due to decreasing crude oil supplies and the scarcity and escalating cost of surfactants has generated research into non-conventional surfactants. Among such surfactants, derivatives of lignin, a phenylpropane polymer which comprises about 17 to 30% of wood, have attracted the attention of researchers for a number of years. As early as 1931, U.S. Pat. No. 1,823,440 issued disclosing a method of enhanced oil recovery involving the use of wood sulfite liquor. More recently, lignin sulfonates have been disclosed as sacrificial agents used to inhibit the deposition of the more costly petroleum sulfonates on the formation. See e.g., U.S. Pat. No. 4,006,779. The present invention allows a significant portion of the expensive petroleum sulfonate in a chemical flood to be replaced with an inexpensive non-petroleum based surfactant while still retaining comparable oil recovery efficiency.
The present inventors have found that when alkali lignins have been chemically modified by oxidation to be soluble at neutral pH, they can be used effectively in enhanced oil recovery surfactant systems. Other workers have described the use of sulfonated alkali lignins and oxidized sulfonated alkali lignins as sacrificial agents for enhanced oil recovery. See, U.S. Pat. Nos. 4,006,779, 4,133,385 and 4,196,777. The prior art does not describe the utility of oxidized alkali lignins in enhanced oil recovery.